Rainfall pulse response of carbon exchange to the timing of natural intra-annual rainfall in a temperate grass ecosystem

Rainfall pulses play a key role in dominating the ecosystem carbon balance in arid and semiarid regions. In the future, the intensity and frequency of rainfall events will be significantly altered. Therefore, discussing the response of the carbon balance to rainfall pulses is extremely important for predicting the effect of global change on the carbon cycle. In grasslands, previous studies have documented that there is a threshold of rainfall size, above which net ecosystem exchange (NEE) would shift from positive (carbon source) to negative (carbon sink). However, does this threshold exist under altered intra-annual rainfall timing during growing season? To answer this question, we conducted an experiment in a typical grassland ecosystem to detect the relationship between rainfall pulse and carbon balance during the growing seasons from 2012 to 2015. Our studies showed that there was no rainfall threshold for the NEE from carbon source to sink with the change in rainfall timing. In the early growing season, there was a positive relationship (carbon source) between the NEE and rainfall pulses. In the middle growing season, there was a negative relationship (carbon sink) between the NEE and rainfall pulses. Moreover, the response magnitude of the NEE was positively related to the activity duration differences between ecosystem respiration (ER) and gross primary productivity (GPP). This study provided a new perspective on the proximate causes of the effects of rainfall on NEE. The mechanism underlying the response patterns of NEE to rainfall pulses is of critical importance for improving our ability to project future changes in the Earth system.